A few are available in the channel. This might be worthwhile at about $20 or so ...
A few notes from very helpful usenet folks on the USB cards ,
... If you want to save time just stop in at the computer section of Circuit City and pick up the ADS 2 port USB card. It's in a red yellow and white box. It works with the Apple USB software 1.4.1 and OS 8.6. I bought one last November for $15.00.
... that would be the 20-ish dollar KeySpan USB card, then... Plenty cheap,
works under 8.6, haven't had a problem with mine yet...
... currently, my rodent, CD burner, and memory-card reader
are the only things using USB in my installation.
... I put one of those in my PowerMac 8600 about 3 years ago, and it is
still going strong. It works with every USB device I ever threw at it,
and I bought it in the first place to use a USB printer.
I don't think that Apple provided drivers for these cards until OS 9,
and the software is still available for download at Apple's site. It's
called "USB Card Support", and it's at version 1.4.1 
Since I bought my Keyspan USB card when I was using OS 8.6, Keyspan
provided drivers for their card so it would work with OS 8.6. Once OS 9
came out and those drivers no longer worked, Apple's drivers were just
released and worked perfectly with the card. You can check to see if
those drivers will work with OS 8.6.
BTW-USB 1 CD writers are so slow, you'd always be burning at 1 or 2xs
speeds. They don't write any faster than that.
From Apple  (1.4.1)
USB Adapter Card Support requires a PowerPC(TM)-based Macintosh computer with PCI or CardBus slots, Mac OS 8.6 or later, and a USB adapter card.
This software cannot be installed on Macintosh computers with built-in USB ports. These computers already include support for built-in USB ports and USB PCI adapter cards.
Important: Mac OS versions 9.1 and later include a later version of this software. Installing USB Adapter Card Support 1.4.1 over Mac OS 9.1, 9.2 or later could result in the USB adapter card not working.
Important: This software supports only USB cards that comply with the Open Host Controller Interface (OHCI) specification and may not work with every USB adapter card.
 http://www.versiontracker.com/dyn/moreinfo/macos/435 and